Using the PXI-4110 or the PXI-4130 for Low Level Resistance Measurements with a DMM

A DMM can take very accurate resistance measurements in a large range. In smaller ranges, however, the accuracy of a resistance measurement taken with a DMM can decrease because noise has a greater effect on small test voltages and the resolution of the DMM is not used as effectively. Use the PXI-4110 or the PXI-4130 along with a DMM, such as the PXI-4071, to increase the accuracy when taking resistance measurements less than 1 Ohm.

Connection Diagram

Figure 1: Connection diagram for using PXI-4110 or PXI-4130 with PXI-4071 to accurately measure low resistances

How the Example Works

This example demonstrates how to take accurate resistance measurements in the mOhm and uOhm range. To take these measurements, the PXI-4110 or the PXI-4130 is used to supply a larger test current than the DMM alone can source.  A larger test current results in a larger voltage across the resistance.  Therefore, we can measure a larger voltage more accurately because it is less susceptible to noise and is digitized by more bits on the DMM's ADC.

By configuring the PXI-4110 as a current source, it can supply a test current to pass through a resistance.  After the DMM takes a voltage measurement, the resistance can be calculated by dividing the current from the voltage (R=V/I). 

Note:  Careful consideration should be made to the maximum power supplied to the DUT (Device Under Test).  Increasing the test current can lead to heating of the DUT which can introduce additional error.  Also, be careful not exceed the power specifications of the DUT.

Normally a DMM can supply its own test current for a resistance measurement, but for the 407x series the maximum test current is 1mA.  This example allows you to take small resistance measurements more accurately than a DMM alone will allow, as long as thermal error is not introduced.  The smallest resistance range of a 407x DMM is 100 Ohms.  This range has a test current of 1mA.  Taking resistance measurements in the mOhm range will not be as accurate for a few reasons.  First of all, a 1mOhm measurement with a 1mA test current results in a voltage of 1uV.  This small voltage can only be represented using a fraction of the bits of the board's ADC.  Second, a small voltage is more susceptible to noise.

To get around this problem we can increase the test current for our resistance measurement.  By using the PXI-4110 or the PXI-4130 we can increase the test current from the 407x's maximum of 1mA to 10mA or greater, which will allow more accurate resistance measurements under 1 Ohm.  This is because applying a larger test current results in a larger voltage that can be measured with better precision by the DMM. 

Related Links

• Precision Power Supply, Guard and Current Amplifier Demos
• NI PXI Programmable Power Supplies and Precision Sources
• Digital Multimeter (DMM) and LCR Meter

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